As semiconductor devices such as a thin film transistor (TFT) are developed, information processing devices are developed which can process more data within a short time. Display devices for displaying the processed data are also in rapid progress.
Examples of the display devices include a liquid crystal display device (LCD), an organic light emitting device (OLED), and a plasma display panel (PDP).
The display devices commonly include TFTs so as to display full-color images. Specifically, display devices having low temperature polysilicon (LTPS) TFTs have been recently introduced.
In the LTPS technology, a channel layer of a TFT for an active matrix display device is formed of polysilicon having higher electron mobility than amorphous silicon. Also, since a driving circuit for controlling a display device can be directly formed on a display substrate, it is unnecessary to arrange separate drive ICs around a display panel. Therefore, the number of components can be reduced compared with a display device using amorphous silicon. The LTPS fabrication technology can provide a display device having characteristics of high durability, slimmer profile, higher brightness, and low power consumption.
In the LTPS TFT, a polysilicon pattern is directly formed on a display substrate, and a gate electrode is formed on the polysilicon pattern. A source electrode and a drain electrode are formed on the polysilicon pattern. A contact hole is formed on an insulating layer between the polysilicon pattern and the gate electrode. The source electrode and the drain electrode are electrically connected to the polysilicon pattern through the contact hole.
However, metal ions of the source electrode and the drain electrode are diffused from the source electrode and the drain electrode to the polysilicon pattern. Especially, after forming a passivation layer on the substrate where an annealing process is performed for remove hydrogen included the passivation layer. In this process, because of the temperature of the annealing process is about 200° C. to 400° C., metal ions or atoms of the source electrode and the drain electrode are diffused from the source electrode and the drain electrode to the polysilicon pattern. Consequently, the length of the polysilicon pattern is gradually narrowed by the diffusion of the metal ions. In this case, the performance of the TFT is greatly degraded and the display quality of the image generated from the display device is also degraded.